The present invention relates to a color cathode ray tube of the type which may be incorporated in a color monitor or in a color TV set; and, more particularly, the invention relates to a color cathode ray tube which decreases the occurrence of beam landing error caused by the movement of a shadow mask structure resulting from a rise in the temperature in the set or a rise in the temperature of the shadow mask, when the color monitor or the color TV set is operated.
A color cathode ray tube is generally constituted by a panel portion, which forms a picture screen, a neck portion for housing an electron gun, and a funnel portion for connecting the panel portion to the neck portion. On the funnel portion, there is provided a deflection device for scanning an electron beam, emitted from an electron gun, on a fluorescent screen formed on the inner surface of the panel.
FIG. 1 is a diagram schematically illustrating the basic structure of a cathode ray tube, which includes a panel 1, a funnel 2, a neck portion 3, a fluorescent screen (screen) 4, a shadow mask structure 5, panel pins 51 for supporting the shadow mask structure, a magnetic shield 6, deflection yokes 7, a magnet 8 for adjusting the purity, a magnet 9 for adjusting the center beam static convergence, a magnet 10 for adjusting the side beam static convergence, and an electron gun 11 which produces electron beams B.
The electron beams for R (red), G (green) and B (blue) colors are deflected in the horizontal direction and in the vertical direction by the deflection device (yokes), provided on the funnel portion, on the way from the electron gun to the fluorescent screen, are selected depending upon the colors by the shadow mask disposed in the panel portion, and impinge upon the fluorescent screen, whereby the fluorescent screen emits light in different colors so that an image is formed on the fluorescent screen.
FIG. 2 is a diagram schematically illustrating the shadow mask structure which comprises a shadow mask 12 having a plurality of electron beam passing openings for selecting colors, a support frame 13 for holding the shadow mask 12, and mask springs 14 for holding the support frame 13 in the panel. The shadow mask structure 5 is held by joining the mask spring-holding holes 141 to the panel pins 51 formed on the panel. Furthermore, the mask spring-holding holes 141 are positioned on a vertical axis or on a horizontal axis that passes nearly through the center of the shadow mask structure 5.
In general, the shadow mask 12 is made of invar (e.g., having a coefficient of thermal expansion of 6.9.times.10.sup.-6 /.degree.C.), the support frame 13 is made of a steel (e.g., having a coefficient of thermal expansion of 1.15.times.10.sup.-5 /.degree.C.), and the mask springs 14 are made of a stainless steel (e.g., having a coefficient of thermal expansion of 1.04.times.10.sup.-5 /.degree.C.). Hereinafter, the term coefficient of thermal expansion refers to the coefficient of linear thermal expansion. In this case, the shadow mask 12 which is nearly flat, suppresses the doming of the shadow mask having low thermal expansion of the invar. In order to decrease the change with time of beam landing in the case of full-luster display, furthermore, the mask springs 14 are often made of a single material without a bimetal function.
When the cathode ray tube incorporated in a color monitor or in a color TV set (hereinafter referred to as the set) is operated, the temperature in the set containing the funnel portion and the neck portion gradually rise, due to heat energy generated by the circuit components in the set, and eventually reaches an equilibrium. Since the screen of the panel is exposed, it has a temperature lower than the temperature inside the set. The heat energy generated by the circuit components in the set raises the temperature in the set and, then, raises the temperature of the funnel. Moreover, the temperature of the inner shield is raised due to radiant heat, causing the temperatures of the support frame and the mask springs to be raised, too.
The temperature surrounding the cathode ray tube is lower at the panel portion than the funnel portion. Thus, the temperature of the panel portion is lower than the temperature of the funnel portion. Therefore, the mask springs joined to the panel pins buried in the panel are heated to a lesser degree than the support frame and are not thermally expanded by the same amount when the mask springs and the support frame have the same coefficient of thermal expansion.
For example, a mask spring support point 141 on the short side or on the long side of the shadow mask structure and a point 131 on the support frame in the vicinity thereof are on the same straight line as a mask spring support point 141 on the opposite side of the mask with respect to the above-mentioned mask spring support point 141 and a point 131 on the support frame in the vicinity thereof. When their positional relationship is maintained, the shadow mask is not distorted. Actually, however, the mask springs and the support frame are not thermally expanded by the same amount, causing the shadow mask structure to be distorted. Distortion in the shadow mask structure causes beam landing shift, deteriorating the color purity.
FIG. 3 illustrates by arrows the motion of points 131 on the support frame near the mask spring support points 141 in the four-pin type shadow mask structure in which the mask springs have a coefficient of thermal expansion nearly equal to that of the support frame, i.e., in which the amount of thermal expansion of the mask springs is smaller than the amount of thermal expansion of the support frame. As described above, the motion of points 131 on the support frame near the mask spring support points 141 is caused by the difference in the thermal expansion between the mask springs 14 and the support frame 13 as a result of an increase in the temperature in the set. In the four-pin type shadow mask structure, the points 131 move in the directions of the arrows; i.e., the shadow mask as a whole is subjected to a rotational force.
FIG. 4 is a diagram illustrating the motion of points 131 on the support frame near the mask spring support points 141 of a three-pin type shadow mask structure, which occurs when the thermal expansion of the mask springs is smaller than the thermal expansion of the support frame, and in which the points 131 move in the directions of the arrows. In the three-pin type shadow mask structure, therefore, the points 131 move in the directions of the arrows, and the force is concentrated on the right upper corner portion of the shadow mask. FIG. 5 shows the directions of shift of the electron beam landing that occurs when a cathode ray tube using the three-pin type shadow mask structure shown in FIG. 4 is mounted on a color TV set.
In general, the mask springs and the support frame are designed by taking into consideration the heat energy that is generated at the support points when the electron beams impinge upon the shadow mask, but without taking into consideration the additional heat energy generated by the circuit components in the set. In a color display tube of the type used for a color monitor, in particular, the structure of the fluorescent screen is of the dot type and involves a stricter problem in regard to color purity than that of the fluorescent screen structure of the stripe type.
In a high definition color display tube in which the shadow mask for determining the dot pitch of the fluorescent screen has a hole pitch of smaller than 0.31 mm, furthermore, this becomes a more serious problem. Besides, in the color display tube, the number of the horizontal scanning lines must be increased. Therefore, the horizontal deflection frequency increases due to the deflection yokes, and an increased amount of heat is generated by the circuit components in the deflection yokes and in the set. The problem of heat generation becomes conspicuous, particularly in a high definition display in which the number of the horizontal scanning lines substantially exceeds 1000. The above-mentioned problem becomes serious, particularly in a high definition color display tube.